[99mTc]-methylene disphosphonate (MDP) planar or single-photon emission computerized tomography (SPECT) bone imaging is one of the most commonly performed nuclear medicine procedures to evaluate bone disorders, such as infection (osteomyelitis), noninfectious inflammation (arthritis), trauma, metabolic bone disease, benign and malignant neoplasms, and metastasis. Nevertheless, concerns are expressed about recurring shortages of 99mTc, which may limit the availability of this imaging agent for routine clinical use. Recently, [18F]NaF in conjunction with PET has been approved for the clinical evaluation of patients with known or suspected bone metastases. Iagaru A, et al., Clin. Nucl. Med. 38:e290-6 (2013); Jadvar H, et al., Semin. Nucl. Med. 45:58-65 (2015). There is currently an increasing number of regional commercial distribution centers for PET radiotracers, thus improving the availability of [18F]NaF (t1/2 110 min, 97% β+, 0.63 MeV max energy) for routine clinical practice.
68Ge/68Ga generators for PET imaging are becoming increasingly available in nuclear medicine clinics. Velikyan I., J. Label. Compd. Radiopharm. DOI: 10.1002/jlcr.3250 (published online Feb. 17, 2015). There are several advantages associated with using 68Ga: 1) A long-lived parent isotope, germanium-68 (68Ge) (t1/2 271 d), allows for an easy and widespread generator distribution; 2) The physical properties of 68Ga (t1/2 68 min, 89% β+, 1.90 MeV max energy) are highly suitable for PET imaging; 3) 68Ge/68Ga generators provide a convenient mechanism for position emitting isotope production without the need for a nearby cyclotron. An important factor to consider is that the emitting β+ energy for 18F and 68Ga is 0.63 MeV and 1.90 MeV, respectively. However, despite the difference in the β+ energy, 18F and 68Ga radiopharmaceuticals exhibit similar spatial resolution, sensitivity, image contrast, and activity recovery coefficients in human tissue, and they produce comparable clinical images in humans.
Due to the relatively short physical half-life of 68Ga and its potential for binding to the blood component transferrin, several essential properties for 68Ga radiopharmaceuticals are needed: 1) The 68Ga complexes should display high in vitro stability; 2) The formation of 68Ga complexes should be kinetically fast; 3)68Ga complexes should be able to form bifunctional molecules for targeting, pre-conjugation, to biologically active molecules; and 4) 68Ga complexes should display suitable in vivo stability in blood circulation with minimal transferrin exchange.
Currently, the most common 68Ga labeled radiopharmaceuticals evaluated are [68Ga]DOTATOC, [68Ga]DOTATATE, and [68Ga]DOTANOC. These compounds are mainly used for detecting the over-expression of somatostatin receptors associated with neuroendocrine tumors. This has attracted significant attention for using PET imaging in the diagnosis of neuroendocrine tumor and various diseases. Morgat C. et al., Gallium-68: chemistry and radiolabeled peptides exploring different oncogenic pathways, Cancer Biother. Radiopharm. 28:85-97 (2013); Sandstrom M, et al. J. Nucl. Med. 54:1755-9 (2013); Velikyan I, et al., Quantitative and qualitative intrapatient comparison of 68Ga-DOTATOC and 68Ga-DOTATATE: net uptake rate for accurate quantification, J. Nucl. Med. 55:204-10 (2014).
A number of Ga complexes have been reported, and they are usually macrocyclic or acyclic polyaza carboxylic acids. These complexes often include metal-chelating ligands designed to form gadolinium (Gd) complexes for use as magnetic resonance imaging (MIRI) contrast agents, such as: diethylenetriaminepentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), and related derivatives (Table 1). Many of these ligands are commonly employed to chelate radioactive metal ions. These include single photon emitting isotopes for SPECT imaging—67Ga, 99mTc, and 111In, as well as positron emitting isotopes for PET imaging—64Cu, 86Y, 89Zr, 68Ga, and 89Sr. Literature reports on polyaza carboxylic acids such as DOTA and related ligands, suggest that they form highly thermodynamic stable complexes with Ga(III). Nevertheless, the complexation of no-carrier-added (n.c.a.) 68Ga with DOTA derivatives has been shown to be inefficient, often requiring heating of 80-100° C. The formation of DOTA ligands with Ga(III) is more sensitive to experimental conditions than that of NOTA analogs. It is likely that the smaller cavity created by the NOTA derivatives fits tighter to the ionic radius of Ga(III). NOTA derivatives, especially 1-(1,3-carboxypropyl)-4,7-carboxymethyl-1,4,7-triazacyclononane (NODAGA), were shown to be more suitable for chelating the Ga(III) ion than DOTA derivatives. Price E. W. and Orvig C., Chem. Soc. Rev. 43:260-90 (2014); Oxboel J., et al., Nucl. Med. Biol. 41:259-67 (2014). The Ga(III)NODAGA complexes exhibited much higher thermodynamic stability as well as rapid complex kinetics. As Ga(III) is a small ion and generally requires an octahedral coordination sphere, Ga(III)NODAGA analogs provide optimal in vitro and in vivo stability. There are several reports in which Ga(III)NODAGA was preferentially chosen as the chelating group in producing bifunctional imaging agents. By using DOTA and NOTA derivatives, many 68Ga labeled bisphosphonates were prepared and tested for bone imaging. It was reported that a bisphosphonate DOTA derivative, [68Ga] 4-{[(bis-phosphonomethyl) carbomoyl]methyl}-7,10-bis-(carboxy-methyl)-1,4,7,10-tetraazacyclododec-1-yl)-acetic acid (BPAMD), displayed good bone uptake and retention in humans. Fellner M., et al., Eur. J. Nucl. Med. Mol. Imaging 37:834 (2010).
Table 1, depicts the structures of bisphosphonates that are reported to be capable of complexing 68Ga for bone imaging. These include ethylene-diamino-N,N,N′,N′-tetrakis-methylene-phosphoric acid (EDTMP), (4-{[(bis-phosphonomethyl)carbomoyl]methyl}-7,10-bis-(carboxy-methyl)-1,4,7,10-tetraazacyclododec-1-yl)-acetic acid (BPAMD), (4-{[(bis-phosphonopropyl)carbomoyl]methyl}-7,10-bis-(carboxymethyl)-1,4,7,10-tetraazacyclododec-1-yl)-acetic acid (BPAPD), tetraethyl-10-{[(2,2-bis-phosphonoethyl)-hydroxyl phosphoryl]methyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (BPPED or DO3ABP)), (4-{[(bis-phosphonopropyl)carbomoyl]hydroxylmethyl}-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA-BP), 2,2′-(7-(((2,2-diphosphonoethyl)(hydroxy)phosphoryl)methyl)-1,4,7-triazo-nane-1,4-diyl)diacetic acid (NO2APBP), 4-{[(bis-phosphonopropyl) carbomoyl]methyl}-1,4,7-triaza cyclonone-1,4-diacetic acid (NOTAMBP), 1,4,7-triazacyclononane-N,Nne-1,tris(bis-phosphonopropyl) carbomoyl]methyl-methylenephosphonic) acid (TRAP(NOTP)), and 1,4,7-triazacyclononane-1,4,7-tri[methylene phosphinic acid] (TRAP(MDP)3). The DOTA and NOTA based bisphosphonates, 68Ga labeled BPAMD and NO2APBP, have been successfully tested in humans as bone-imaging agents.
TABLE 1Bisphosphonates that Can Complex 68Ga for Bone Imaging       
Several chelating groups reported for complexing Ga(III) are: DOTA, 1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid] (TRAP (NOPO)), cyclohexyl-1,2-[[6-carboxy-pyridin-2-yl]-methyl amino] ethane (H2CHX DEDPA), and (5S,8S,22S,26S)-1-amino-5,8-dibenzyl-4,7,10,19,24-pentaoxo-3,6,9,18,23,25-hexaazaoctacosane-22,26,28-tri-carboxylic acid trifluoroacetate (CHX-A″-DTPA-DUPA-Pep). See Simecek J., et al., Chem. Med. Chem. 8:95-103 (2013); Ramogida C. F., et al., Inorg. Chem. 54:2017-31 (2015); Baur B., et al., Pharmaceuticals (Basel) 7:517-29 (2014).
Prostate-specific membrane antigen (PSMA) is a highly specific prostate epithelial cell membrane antigen. Many reports suggest that PSMA is highly expressed in various tumors, including prostate cancer. Often, PSMA expression increases in higher-grade cancers and metastatic diseases. In a majority of neovasculature in solid tumors, there is high expression of PSMA, but not in normal vasculature. This makes PSMA a suitable target for cancer detection and therapy. Certain Ga-prostate specific membrane antigen (PSMA) tagged complexes showed high-affinity binding and effective targeting of PSMA-expressing tumor models in vitro. Two studied agents for imaging PSMA binding sites in cancer patients are [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer), and its related dimer, [68Ga](Glu-NH—CO—NH-Lys(Ahx))2-HBED-CC. Both complexes were prepared and were reported to show high PSMA binding as seen in Table 2. Baur B., et al., Pharmaceuticals (Basel) 7:517-29 (2014); Schafer M., et al., EJNMMI Res 2:23 (2012); Eder M., et al., Pharmaceuticals (Basel) 7:779-96 (2014); Eder M., et al., Bioconjug. Chem. 23:688-97 (2012). Although both [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer) and [68Ga](Glu-NH—CO—NH-Lys(Ahx))2-HBED-CC (dimer) exhibited comparable preclinical data, the current PSMA/PET imaging agent of choice for human study is the monomer. It is generally accepted that Glu-NH—CO—NH-Lys(Ahx)-provides high binding affinity to the PSMA receptors on the cell membrane of tumors.
TABLE 2Proposed structures of PSMA targeting imaging agents[68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer) and [68Ga](Glu-NH—CO—NH-Lys(Ahx))2-HBED-CC (dimer). 
Most clinical studies to date have been performed with [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer). Using HBED instead of commonly employed DOTA and NOTA, as a ligand for chelating Ga(III) has certain advantages. Stability constants (log Kd) for Ga(III)-DOTA and Ga(III)—NOTA complexes were previously reported (log Kd=21.3 and 31.0, respectively). Compared to DOTA and NOTA, the HBED chelating group forms a stronger, more stable Ga(III) complex: a log Kd value of 38.5 was reported for Ga(III)-HBED-CC.
A need continues to exist for bone imaging agents that employ available radionuclides, form complexes quickly, are stable in vitro and in vivo, and do not rapidly transfer radionuclide to transferrin in the bloodstream.